1. Field of the Invention
The present invention relates generally to a multi-carrier transmission system and a transmission method. More particularly, the invention relates to a multi-carrier transmission system and a transmission method known as Discrete Multi-Tone (DMT) modulation system.
2. Description of the Related Art
A typical example of the conventional DMT type multi-carrier transmission system, there has been a technology disclosed in U.S. Pat. No. 5,479,447 to Chow et al.
An Asymmetric Digital Subscriber Line (ADSL) unit to be employed in the DMT system is designed to perform modulation for a plurality of carriers by Quadrature Amplitude Modulation (QAM) and to transmit the modulated carriers by multiplexing using Inverse Fast Fourier Transform (IFFT). On the receiver side, each carrier is extracted from the multiplexed reception signal using FFT to demodulate into the QAM modulated signal.
In such case, a signal to noise ratio (SNR) of each carrier is measured for bit distribution for each of a plurality of carriers, and the bit distribution is derived according to the measured SNR. For example, as identified by 15 in FIG. 13, the frequency on the horizontal axis is each carrier to be used or transmission. A frequency band of each carrier is 4.3125 Hz and total number of the frequency bands is 256. Each of these values is not specific and limitative. Upon data transmission, the carriers are modulated respectively. At this time, a value of SNR is evaluated to derive bit distribution according to the evaluated SNR. In this case, in evaluation of SNR, respective SNR values are derived at respective frequency bands of the carriers.
Each carrier performs transmission at a bit number according to each of bit distributions thus determined. The bit number is determined on the basis of the evaluated SNR so that a given transmission speed can be satisfied and a performance margin becomes maximum.
One example of a method for calculating the bit distribution to provide the maximum performance margin in the transmission speed in the conventional DMT type ADSL technology, has been disclosed in the above-identified U.S. Pat. No. 5,479,447. FIG. 13 illustrates one example of the bit distribution method set forth above. Once, a desired transmission speed (bit rate) is given, number of bits is distributed for each carrier (16) so that each carrier may have the maximum performance margin on the basis of the SNR value (15) of the measured transmission path.
In the DMT type ADSL technology, in Japan, a Time Compression Multiplexing (TCM) type ISDN is present within the same cable. A periodic crosstalk caused by presence of the TCM type ISDN causes substantial noise in a signal for the ADSL. Here, discussion will be given for crosstalk to be generated when ADSL line and TCM-ISDN line are commonly present in the same cable, with reference to FIG. 14. FIG. 14 illustrates crosstalk to be caused by ADSL Termination Unit-Remote side (ATU-R) due to data transmission by the TCM-ISDN line while data transmission is performed from a down direction (a direction of ADSL Termination Unit-Center side (ATU-C)) to the ATU-R in the ADSL line.
As shown in FIG. 14, while data transmission in the down direction is performed in the ADSL line, if data transmission in the down direction is performed by the TCM-ISDN line, Far End Crosstalk (FEXT) is caused. On the other hand, if data transmission in an up direction is performed the TCM-ISDN line while data transmission in the down-direction is performed in the ADSL line, Near End Crosstalk (NEXT) is caused. In the TCM type ISDN line, data transmission is performed alternately in up direction and down direction. Therefore, due to influence of Ping-Pong type data transmission of the ISDN line, near end crosstalk and far end crosstalk are caused periodically in the ADSL line.
When communication is performed by the conventional ADSL technology, due to periodic crosstalk, large amount of error is caused at the occurrence of near end crosstalk (NEXT) in bad noise condition. On the other hand, when the transmission speed is calculated adapting to communication under NEXT noise, the transmission speed is lowered significantly. Under a condition where a crosstalk noise from the ISDN is present, so-called dual bit map system is considered for improving communication performance of the ADSL unit. In this system, ADSL unit employs a system which has two bit maps (bit distributions) and changes communication speeds by switching the bit map in synchronism with the period of the crosstalk noise. Upon occurrence of the far end crosstalk (FEXT), communication speed is set high for low noise level, and upon occurrence of NEXT, communication speed is set low for high noise level.
However, since a plurality of the SNR values of the transmission line in the dual bit map system are present, it is not possible to perform bit distribution from the bit rate (transmission speed) given from the larger level. Namely, it is necessary that, on the basis of the measured SNR values, the given bit rate is distributed to two kinds of transmission speed and bit number is distributed for respective carriers.
For the problems set forth above, if noise amount on the line is periodically varied and a plurality of transmission speed is switched in synchronism with variation of the noise, maximum performance margin cannot be obtained by the conventional bit distribution method.
It is an object of the present invention to provide a multi-carrier transmission system and a method therefor employing a bit distribution method capable of solving the problem set forth above by regarding a plurality of SNR values evaluated at different timings as one SNR value evaluated at different frequency at the same timing.
Another object of the present invention to provide a multi-carrier transmission system and a method therefor employing a bit distribution method capable of solving the problem set forth above by realizing a given transmission speed depending upon a plurality of SNR values evaluated at different timings and thus maximizing performance margin.
According to the first aspect of the present invention, a multi-carrier transmission system performing data transmission between first and second communication stations under presence of a plurality of periodically varying noises, comprises:
signal to noise ratio evaluating means for obtaining a plurality sets of signal to noise ratio by evaluating signal to noise ratio of each carrier of the multi-carriers at different timing respectively corresponding to a plurality of kinds of noise environments; and
bit distribution means for performing bit distribution of each carrier depending upon one set of signal to noise ratio with taking a plurality of sets of signal to noise ratios as one set of signal to noise ratio evaluated on different frequency at the same timing.
In the construction set forth above, the signal to noise ratio evaluating means may be constructed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments varies in regular interval period, and the bit distribution means may be constructed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. On the other hand, the signal to noise ratio evaluating means may be constructed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments varies irregular interval period, and the bit distribution means may be constructed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. Also, the bit distribution means may perform bit distribution depending upon the one signal to noise ratio set and a power restriction value of each of the carriers.
According to the second aspect of the present invention, a multi-carrier transmission system performing data transmission between first and second communication stations under presence of a plurality of periodically varying noises, comprises:
signal to noise ratio evaluating means for obtaining a plurality sets of signal to noise ratio by evaluating signal to noise ratio of each carrier of the multi-carrier at different timing respectively corresponding to a plurality of kinds of noise environments; and
bit distribution means for performing bit distribution of each carrier for realizing a given transmission speed and a maximum performance margin depending upon respective values of a plurality of signal to noise ratio sets.
In the construction set forth above, the signal to noise ratio evaluating means may be constructed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments vary in regular interval period, and the bit distribution means may be constructed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. On the other hand, the signal to noise ratio evaluating means may be constructed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments vary irregular interval period, and the bit distribution means may be constructed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. The bit distribution means may perform bit distribution depending upon the one signal to noise ratio set and a total transmission power restriction value.
In case of data transmission from the first communication station to the second communication station, the first communication station may include means for transmitting predetermined plurality of transmission speed to the second communication station and the second communication station may have the signal to noise ratio evaluating means and the bit distribution means, the bit distribution means may include means for deriving a margin in data transmission on the basis of a plurality of transmission speed transmitted from the first communication station, means for selecting an optimal transmission speed from the plurality of transmission speed on the basis of the derived margin, and means for deriving bit distribution of each of the carriers according to the selected transmission speed. The second communication station may include means for transmitting the bit distribution to the first communication station, and the first communication station may performs data transmission to the second communication station according to the bit distribution. Noise source of the two kinds of noises may be present on a common cable with a communication line between the first and second communication stations. The two kinds of noise environments may be a first noise environment and a second noise environment having worse noise condition than the first noise environment. The two kinds of noises may be caused by far end crosstalk and near end crosstalk. A communication line for data transmission between the first and second communication stations may be a digital subscriber line.
According to the third aspect of the present invention, a multi-carrier transmission method performing data transmission between first and second communication stations under presence of a plurality of periodically varying noises, comprises:
signal to noise ratio evaluating step of obtaining a plurality sets of signal to noise ratio by evaluating signal to noise ratio of each carrier of the multi-carriers at different timing respectively corresponding to a plurality of kinds of noise environments; and
bit distribution step of performing bit distribution of each carrier depending upon one set of signal to noise ratio with taking a plurality of sets of signal to noise ratios as one set of signal to noise ratio evaluated on different frequency at the same timing.
In the method set forth above, the signal to noise ratio evaluating step may be designed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments vary in regular interval period, and the bit distribution step may be designed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. The signal to noise ratio evaluating step may be designed to derive the set of the signal to noise ratio corresponding to each of two kinds of noise environments when two kinds of noise environments vary irregular interval period, and the bit distribution step may be designed for establishing the bit distribution with taking two kinds of signal to noise ratio sets as the one signal to noise ratio set. The bit distribution step may perform bit distribution depending upon the one signal to noise ratio set and a power restriction value of each of the carriers.
According to the fourth aspect of the present invention, a multi-carrier transmission method performing data transmission between first and second communication stations under presence of a plurality of periodically varying noises, comprises:
signal to noise ratio evaluating step of obtaining a plurality sets of signal to noise ratio by evaluating signal to noise ratio of each carrier of the multi-carriers at different timing respectively corresponding to a plurality of kinds of noise environments; and
bit distribution step of performing bit distribution of each carrier for realizing a given transmission speed and a maximum performance margin depending upon respective values of a plurality of signal to noise ratio sets.
The second communication station may include step of transmitting the bit distribution to the first communication station, and the first communication station may perform data transmission to the second communication station according to the bit distribution.